In a tube-bundle and other structures wherein a multiplicity of tubes are anchored in a pressure tight manner in a plate, e.g. a tube sheet, the tube can have an end which is fitted into a respective bore of the tube sheet and which is expanded so that the engagement of the outer surface of the tube end and the inner surface of the sheet defining the bore are in a press-fitted, mechanically secure engagement preventing the leakage of fluids even under elevated pressures between these surfaces.
Tube bundles and tube sheets of this type can be widely used in tube-bundle heat exchangers, boilers, reactors, condensers, digestors and like equipment.
It is known to expand a tube end in a bore of a tube sheet by hydraulic pressure, i.e. by introducing a mandrel into the tube end so that the mandrel defines with this tube end a compartment into which a liquid can be fed, the liquid being pressurized to spread the tube end into engagement with the wall of the bore.
For this purpose, a device for hydraulically expanding a tube end can comprise the aforementioned mandrel and means for delivering the extraction fluid to the clearance between the mandrel and the tube end, and a high pressure source for pressurizing this liquid.
This source can include a piston or ram which is displaced by another fluid and, by its displacement, sharply increases the pressure of the expansion liquid. Reference may be had to various publications describing the hydraulic expansion of tubes, for example the German Patent Document De OS No. 19 39 105 and an article by M. Podhorsky and H. Krips entitled Hydraulisches Aufweiten von Rohren (Hydraulic Expansion of Tubes) in VGB Kraftwerkstechnik, No. 1, 1979, pp 81-87.
In these publications, it is described how the tube end can be fitted with play in a bore of a tube sheet, a pressurizing mandrel can be inserted into the tubed end, the clearance between the mandrel and the tube end sealed at two spaced-apart locations to define an expansion zone, and a liquid (pressurizing fluid) introduced into this clearance.
This fluid, upon pressurization, spreads the tube end radially. The pressure-tight attachment of tubes in tubed sheets or the like, for example for tube bundle sheets exchangers, is of considerable significance for the economic fabrication of such equipment in an efficient and reproducible manner.
For some decades, mechanical press fitting techniques have been used although problems have been encountered with such techniques because they have been comparatively expensive and did not always yield reproducible results. The problems with such mechanical techniques are described in the above-mentioned Podhorsky et al article which also discusses the ability of hydraulic expansion approaches to overcome the disadvantages of the earlier systems.
In the Podhorsky et al article and in German Patent Document DE-AS No. 26 16 523, the pressurizing liquid is water while the force-generating liquid is another hydraulic medium, e.g. oil which is supplied at high pressure by a pump.
Between the pressurizable liquid and this expansion-generating liquid, a piston or ram is provided so that an alternatively small cross section of the ram is applied to the liquid in the expansion zone while the oil of the pump applies its force to a relatively large cross section portion of the piston or ram so that the piston acts as a pressure multiplier.
The piston or ram can be considered to have two piston members as in a stepped piston, one of which receives the pressure of the oil while the other pressurizes the water.
One of the disadvantages of the earlier systems of this type (see especially the German Patent Document DE-AS No. 26 16 523) is that means was required to fill the clearance with the pressurizable liquid and that between this means and the expansion zone, an automatically operated check valve was required so that the pressurizable liquid could be supplied but loss of force upon hydraulic pressurization was avoided, and during the application of the high pressure displacement of the pressurizing liquid back to the latter means was precluded.
A check valve of this type had various problems associated with its use. For example, pressures as high as 4,00 bar were required to expand the tube ends. The volume of the pressurizing liquid under the expansion pressure is less than the volume of the expanded liquid by the compression volume. Upon expansion of the tube end, a displacement volume is created which tends to drop the expansion pressure. However, since the compression volume as a rule is larger than the displacement value, even when the applied pressure is reduced, in a passive state of the system, the residual pressure in the zone remains at a higher level and means must be provided to bleed off this pressure excess. Thus the use of the check valve means that the other devices must be provided to bleed off excess pressure.
Furthermore at the levels of expansion pressure used, e.g. about 4,000 bar, it is not possible to provide absolutely pressure-tight check valves. Furthermore, since the filling of the clearance is generally effected at elevated pressure levels, say 400 bar, rapid and complete closure of the check valve cannot be ensured and frequently the residual pressure in the clearance which can be 300 bar or greater may exceed the filling pressure and prevent reopening of the valve.
In conventional devices utilized heretofore manually operated valves were necessary to bleed off the residual pressure.